1. Field of the Invention
The present invention relates to a magnetostriction type torque sensor.
2. Description of the Prior Art
FIG. 11 illustrates an example of prior art magnetostriction type torque sensors. The magnetostriction type torque sensor comprises magnetostrictive films 91 and 92 formed on outer peripheral surfaces of a rotary shaft 9, excitation coils 93 and 93 disposed around the rotary shaft 9 away from the magnetostriction films 91 and 92 by a predetermined distance in a radial direction, and detection coils 94 and 94 disposed on the excitation coils 93 and 93.
Configurational magnetic anisotropies are given to the configurations of the magnetostrictive films 91 and 92 so that the magnetostrictive films 91 and 92 are subjected to stresses acting in direction being opposite to each other. In a magnetic film having positive magnetostriction, for instance, in the magnetostrictive film 91 subjected to a tensile stress, the magnetic permeability of the magnetostrictive film 91 increases because of the inverse magnetostrictive effect. 0n the other hand, for instance, in the magnetostrictive film 92 subjected to a compression stress, the magnetic permeability of the magnetostrictive film 92 decreases because of the inverse magnetostrictive effect. Hence, the mutual inductances between the excitation coils 93 and the detection coils 94 vary in directions being opposite to each other when an alternating current is applied in the excitation coils 93 and 93, and accordingly the torques of the rotary shaft 9 can be detected from the output voltage differences between the detection coils 94 and 94.
However, when detecting the torques of a rotary shaft 9 made of iron, steel or the like having high magnetic permeability with the prior art magnetostriction type torque sensor, most of the mutual inductances between the excitation coils 93 and the detection coils 94 are generated by the rotary shaft 9 itself, and small mutual inductance components result from magnetic fluxes passing through the magnetostrictive films 91 and 92, namely most of the magnetic fluxes have passed through the rotary shaft 9. Hence, the variation rate of the mutual inductance resulting from the magnetic permeability variation of the magnetostrictive film 92 has been small. As a result, the prior art magnetostriction type torque sensor has been suffering from a problem of low detection sensitivity or S/N ratio (i.e., signal-to-noise ratio).
Likewise, when detecting the torques of a rotary shaft 9 made of iron, steel, aluminum alloy, titanium or the like having high electric conductivity with the prior art magnetostriction type torque sensor, the impedances of the excitation coils 93 decrease because of the eddy current loss resulting from alternating magnetic fluxes penetrating through the rotary shaft 9. Hence, the prior art magnetostriction type torque sensor has again exhibited deteriorated detection sensitivity or S/N ratio.